It has long been the practice of commercial photographers and photo-finishing studios to encode information about individual film negatives on the edge of the film. Encoding marks placed on the edge of the film are typically opaque markings, transparent markings, or machine readable holes punched in the film edge. These markings serve to identify the location of a particular frame of photographic film and may further comprise a pattern which is decodable to provide useful information to the photographer or photo finisher about the corresponding film exposure. With the advent of automated photographic film processing equipment, accurate detection and interpretation of such film edge marks has become more important. Further, increased processing speeds of commercial photographing equipment have strained the abilities of current technology to accurately detect and interpret these marks in the higher speed environments.
Two principal methods have been used for detecting and reading edge markings on photographic film. In one method, utilized solely for punch marked film, differential pressure switches are located on one side of the film strip, and high pressure air is directed to the opposite side of the same film. As the punched portion of the film passes by the differential pressure switch, the pressure from the opposing air activates the switch, causing it to close, thereby indicate the presence of a punched hole. However, differential pressure switch sensors are not capable of reading film that is merely marked. Further, at high speeds, punched holes pass the sensor orifice too quickly for the pressure switch to accurately record the change in pressure.
A second method currently in use is the optical sensor. In this method, a light source is used in combination with a photocell to detect the presence of markings as the film edge is passed between the light source and the photocell. Conventional cadmium sulfide photocells are capable of accurately detecting either holes or marks only when presented a high level of consistency in the photographic film substrate and dyes. Because such photocells have poor response times, however, this method is also limited in its inability to process film at high speed. A further drawback of this methodology is intolerance to variations in film size. Conventional film mark reading devices must be precisely positioned in relation to the film marks, and frequently malfunction if misalignment occurs. This problem is further exacerbated by variations in film tracking, as the strip of film moves through the processing equipment. Because the film is under only the minimum required tension as it moves on rollers, it is not unusual for the edge of the film to track laterally in relation to the conventional sensor.
The invention described in my co-pending U.S. patent application Ser. No. 639,566 entitled "Compensating Optical Sensor" provides an improved optical sensor and mounting means, which allows the sensor to move in relation to the photographic film, and which is dynamically responsive to changes in film density. The invention automatically adapts to variances in the base density of the film, thereby allowing either punched hole or photographic marks on the film to be accurately detected. The circuitry associated with the sensor automatically adjusts the sensitivity of the sensor based on the density of the film, and does so dynamically during film transport. Accordingly, the photographic sensor accurately detects and reads marks or punched holes, even though the photographic density within a single roll of film may vary substantially. No recalibration of the sensor is required when different types of marking, or different types of film are presented to the processing equipment.
In spite of the improvements that I have made in devices adapted to reach such markings disposed along the edge of the photographic film strip, the reading of such marks inherently complicates and slows automated film processing. Further, additional and time consuming steps are required to place the markings on the edge of the photographic film. Thus, the steps necessary for placement, detection and subsequent interpretation of such markings strain the abilities of current technology for automated high speed photographic film processing environments.
One embodiment of the present invention is designed to eliminate, or at least minimize, the necessity for use of markings on the edge of the film to identify individual frames of photographic film. Therefore, the present invention eliminates the necessity for the steps of placement, detection and subsequent interpretation of such markings.
A further embodiment of the present invention is designed to measure the size and optical center of an image in an individual frame of photographic film on a strip of photographic film.